1. Field of the Invention
The present invention is directed, in general, to wireless communication systems and, more specifically, to multiplexing control and data information in single-carrier frequency division multiple access (SC-FDMA) communication systems.
2. Description of the Related Art
In particular, the present invention considers the transmission of positive or negative acknowledgement bits (ACK or NAK, respectively) and channel quality indicator (CQI) bits together with data information bits in an SC-FDMA communications system and is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) long term evolution (LTE). The invention assumes the uplink (UL) communication corresponding to the signal transmission from mobile user equipments (UEs) to a serving base station (Node B). A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a base transceiver system (BTS), an access point, or some other terminology. The ACK/NAK bits and CQI bits may also be referred to simply as control information bits.
The ACK or NAK bits are in response to the correct or incorrect, respectively, data packet reception in the downlink (DL) of the communication system, which corresponds to signal transmission from the serving Node B to a UE. The CQI transmitted from a reference UE is intended to inform the serving Node B of the channel conditions the UE experiences for signal reception, enabling the Node B to perform channel-dependent scheduling of DL data packets. Either or both of the ACK/NAK and CQI may be transmitted by a UE in the same transmission time interval (TTI) with data or in a separate TTI with no data. The disclosed invention considers the former case, which may also be referred to as data-associated transmission of the ACK/NAK and/or CQI.
The UEs are assumed to transmit control and data bits over a TTI corresponding to a sub-frame. FIG. 1 illustrates a block diagram of the sub-frame structure 110 assumed in the exemplary embodiment of the disclosed invention. The sub-frame includes two slots. Each slot 120 further includes seven symbols and each symbol 130 further includes of a cyclic prefix (CP) for mitigating interference due to channel propagation effects, as it is known in the art. The signal transmission in the two slots may be in the same part or it may be at two different parts of the operating bandwidth. Furthermore, the middle symbol in each slot carries the transmission of reference signals (RS) 140, also known as pilot signals, which are used for several purposes including for providing channel estimation for coherent demodulation of the received signal.
The transmission bandwidth (BW) is assumed to include frequency resource units, which will be referred to herein as resource blocks (RBs). An exemplary embodiment assumes that each RB includes 12 sub-carriers and UEs are allocated a multiple N of consecutive RBs 150. Nevertheless, the above values are only illustrative and not restrictive to the invention.
An exemplary block diagram of the transmitter functions for SC-FDMA signaling is illustrated in FIG. 2. Coded CQI bits 205 and coded data bits 210 are multiplexed 220. If ACK/NAK bits also need to be multiplexed, the exemplary embodiment assumes that data bits are punctured to accommodate ACK/NAK bits 230. Alternatively, CQI bits (if any) may be punctured or different rate matching, as it is known in the art, may apply to data bits or CQI bits to accommodate ACK/NAK bits. The discrete Fourier transform (DFT) of the combined data bits and control bits is then obtained 240, the sub-carriers 250 corresponding to the assigned transmission bandwidth are selected 255, the inverse fast Fourier transform (IFFT) is performed 260 and finally the cyclic prefix (CP) 270 and filtering 280 are applied to the transmitted signal 290.
Alternatively, as illustrated in FIG. 3, in order to transmit the control (ACK/NAK and/or CQI) bits 310, puncturing of coded data bits 320 may apply 330 (instead of also applying rate matching as in FIG. 2) and certain coded data bits (for example, the parity bits in case of turbo coding) may be replaced by control bits. The discrete Fourier transform (DFT) 340 of the combined bits is then obtained, the sub-carriers 350 corresponding to the assigned transmission bandwidth are selected 355 (localized mapping is assumed but distributed mapping may also be used), the inverse fast Fourier transform (IFFT) 360 is performed and finally the cyclic prefix (CP) 370 and filtering 380 are applied to the transmitted signal 390.
This time division multiplexing (TDM) illustrated in FIG. 2 and FIG. 3 between control (ACK/NAK and/or CQI) bits and data bits prior to the DFT is necessary to preserve the single carrier property of the transmission. Zero padding, as it is known in the art, is assumed to be inserted by a reference UE in sub-carriers used by another UE and in guard sub-carriers (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated in FIG. 2 and FIG. 3. Similarly, the encoding process for the data bits and the CQI bits, as well as the modulation process for all transmitted bits, are well known in the art and are omitted for brevity.
At the receiver, the inverse (complementary) transmitter operations are performed. This is conceptually illustrated in FIG. 4 where the reverse operations of those illustrated in FIG. 2 are performed. As it is known in the art (not shown for brevity), an antenna receives the radio-frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 410 passes through a time windowing unit 420 and the CP is removed 430. Subsequently, the receiver unit applies an FFT 440, selects 445 the sub-carriers 450 used by the transmitter, applies an inverse DFT (IDFT) 460, extracts the ACK/NAK bits and places respective erasures for the data bits 470, and de-multiplexes 480 the data bits 490 and CQI bits 495. As for the transmitter, well known in the art receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity and they are not material to the present invention.
The control bits typically require better reception reliability than the data bits. This is primarily because hybrid-automatic-repeat-request (HARQ) usually applies to data transmission but not to control transmission. Additionally, ACK/NAK bits typically require better reception reliability that CQI bits as erroneous reception of ACK/NAK bits has more detrimental consequences to the overall quality and efficiency of communication than does erroneous reception for the CQI bits.
The size of resources in a transmission sub-frame required for control signaling for a given desired reception reliability depend on the channel conditions the signal transmission from a UE experiences and in particular, on the signal-to-interference and noise ratio (SINR) of the received signal at the serving Node B.
There is a need to determine the placement of control bits when transmitted in the same sub-frame with data bits so that better reception reliability is provided for the control bits than for the data bits.
There is another need to determine the placement of acknowledgement bits relative to channel quality indication bits, in case they are simultaneously multiplexed, in order to provide better reception reliability for the former.
There is another need to dimension the resources required for the transmission of acknowledgement bits, in a sub-frame also containing data bits, as a function of the channel conditions experienced by the signal transmission from a UE.